Best options to go from iPhone 11 Pro to iPhone 18 Pro Max

I have done so much research and have heard so many various things, so maybe I can get a clear answer from someone on here. I want to get the iPhone 18 Pro Max 512 GB. I currently own an iPhone 11 Pro, and it's already paid off and has AppleCare One on it. I was planning on trading the 11 for the 18. Should I buy the new phone directly from Apple, T-Mobile, or Costco? (Or another source/option if there's a better deal somewhere else) I've heard Costco gives the same deal as Apple and T-Mobile, but with an extra $100 Costco gift card/rebate (correct me if I am wrong tho). I have Go5G Plus as my T-Mobile plan and want to keep that plan. Also, should I buy it now, wait for Black Friday, or is there another time in the near future that offers better deals to buy a new iPhone? I'm conflicted and tired of doing more research, so if anyone can offer some advice please!